Catalogue of SPI-ACS Gamma-Ray Burst

The Anti-Coincidence Shield SPI-ACS
The anti-coincidence system of SPI, developed in the Gamma-ray department of the Max-Planck Institute for Extraterrestrial Physics, consists of 91 Bismuth Germanate (BGO) and provides a quasi-omnidirectional field of view with a large (0.3 square meter) effective area for the detection of gamma-ray events between ~80 keV and ~10 MeV. Unfortunately, the electronics impedes a spatial resolution because only the total event rate of all crystals with 50 ms time resolution is telemetered down to Earth. The SPI-ACS data do not includes spectral information because the energy of an interacting gamma-ray photon is not measured. Since December 2002, SPI-ACS has been an important member of the 3rd Interplanetary Network of Gamma-ray burst detectors , which provides burst localizations using the triangulation method. Herewith, the lack of spatial resolution of the ACS can at least partly be countervailed.
The Gamm-Ray Burst Selection
The base sample was constructed from all triggered events in the SPI-ACS overall rate light curve. Each trigger in this preliminary sample was subsequently be checked for particle or solar origin using the INTEGRAL Radiation Monitor (IREM) and the Geostationary Operational Environmental Satellites (GOES), respectively. Triangulated events with a localization consistent with the position of a Soft Gamma-ray Repeater (SGR) were removed from the sample. A significance threshold was then applied to each event with possible cosmic origin, such that a trigger was included in the sample of GRB candidates when its significance, S, above the background, B, exceeds S=12 sigma in any time interval during the event. Here, 1 sigma corresponds to 1.57 sqrt(B), where the factor 1.57 takes into account the measured deviation of the background noise from a Poissonian distribution. The conservative threshold of S=12 sigma has been chosen in order to minimize the contamination by weak events of non-GRB origin.
The Catalogue
According to the selection criteria described above, a total of 374 GRB candidates were detected from Oct. 27, 2002 to Jan. 12, 2005. In addition, 14 GRBs with a significance below the selection threshold but confirmed by other gamma-ray missions, were detected. From the total 388 GRB candidates the cosmic origins of 179 have been confirmed. More information can be found in Rau, von Kienlin, Hurley & Lichti 2005, A&A, 438, 1175.
Events with T90<0.25s
The sample shows the known duration bimodality with a strong excess at the very short end of the distribution (<0.25 s). The origin of this population of events (~40 % of the total number) is significantly different from the normal GRB sample, as shown both by the logN-logCmax distribution as well as by a V/Vmax test. Observations of simultaneous saturations in the spectrometer Ge detectors and very short events in the SPI-ACS overall rate suggest a cosmic ray origin for a significant fraction of these events.

enter catalogue

SPI-ACS triggers at the INTEGRAL Science Data Center
SPI-ACS triggers at the MPE (2003.... up to 2011)
MPE Home

Comments and questions to Arne Rau
May 06, 2005; link update R Diehl Aug 2013